PHP supports eight primitive types. Four scalar types: boolean integer float (floating-point number, aka 'double') string Two compound types: array object And finally two special types: resource NULL This manual also introduces some pseudo-types for readability reasons: mixed number callback You may also find some references to the type "double". Consider double the same as float, the two names exist only for historic reasons. The type of a variable is usually not set by the programmer; rather, it is decided at runtime by PHP depending on the context in which that variable is used. If you want to check out the type and value of a certain expression, use var_dump. If you simply want a human-readable representation of the type for debugging, use gettype. To check for a certain type, do not use gettype, but use the is_type functions. Some examples: ]]> If you would like to force a variable to be converted to a certain type, you may either cast the variable or use the settype function on it. Note that a variable may be evaluated with different values in certain situations, depending on what type it is at the time. For more information, see the section on Type Juggling. Also, you may be interested in viewing the type comparison tables, as they show examples of various type related comparisons. This is the easiest type. A boolean expresses a truth value. It can be either &true; or &false;. The boolean type was introduced in PHP 4. To specify a boolean literal, use either the keyword &true; or &false;. Both are case-insensitive. ]]> Usually you use some kind of operator which returns a boolean value, and then pass it on to a control structure. \n"; } // ...because you can simply type if ($show_separators) { echo "

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\n"; } ?> ]]> To explicitly convert a value to boolean, use either the (bool) or the (boolean) cast. However, in most cases you do not need to use the cast, since a value will be automatically converted if an operator, function or control structure requires a boolean argument. See also Type Juggling. When converting to boolean, the following values are considered &false;: the boolean &false; itself the integer 0 (zero) the float 0.0 (zero) the empty string, and the string "0" an array with zero elements an object with zero member variables (PHP 4 only) the special type NULL (including unset variables) Every other value is considered &true; (including any resource). -1 is considered &true;, like any other non-zero (whether negative or positive) number! ]]> An integer is a number of the set Z = {..., -2, -1, 0, 1, 2, ...}. See also: Arbitrary length integer / GMP, Floating point numbers, and Arbitrary precision / BCMath Integers can be specified in decimal (10-based), hexadecimal (16-based) or octal (8-based) notation, optionally preceded by a sign (- or +). If you use the octal notation, you must precede the number with a 0 (zero), to use hexadecimal notation precede the number with 0x. ]]> Formally the possible structure for integer literals is: The size of an integer is platform-dependent, although a maximum value of about two billion is the usual value (that's 32 bits signed). PHP does not support unsigned integers. If invalid digit is passed to octal integer (i.e. 8 or 9), the rest of the number is ignored. ]]> If you specify a number beyond the bounds of the integer type, it will be interpreted as a float instead. Also, if you perform an operation that results in a number beyond the bounds of the integer type, a float will be returned instead. ]]> Unfortunately, there was a bug in PHP so that this does not always work correctly when there are negative numbers involved. For example: when you do -50000 * $million, the result will be -429496728. However, when both operands are positive there is no problem. This is solved in PHP 4.1.0. There is no integer division operator in PHP. 1/2 yields the float 0.5. You can cast the value to an integer to always round it downwards, or you can use the round function. ]]> To explicitly convert a value to integer, use either the (int) or the (integer) cast. However, in most cases you do not need to use the cast, since a value will be automatically converted if an operator, function or control structure requires an integer argument. You can also convert a value to integer with the function intval. See also type-juggling. &false; will yield 0 (zero), and &true; will yield 1 (one). When converting from float to integer, the number will be rounded towards zero. If the float is beyond the boundaries of integer (usually +/- 2.15e+9 = 2^31), the result is undefined, since the float hasn't got enough precision to give an exact integer result. No warning, not even a notice will be issued in this case! Never cast an unknown fraction to integer, as this can sometimes lead to unexpected results. ]]> See for more information the warning about float-precision. See String conversion to numbers Behaviour of converting to integer is undefined for other types. Currently, the behaviour is the same as if the value was first converted to boolean. However, do not rely on this behaviour, as it can change without notice. Floating point numbers (AKA "floats", "doubles" or "real numbers") can be specified using any of the following syntaxes: ]]> Formally: LNUM [0-9]+ DNUM ([0-9]*[\.]{LNUM}) | ({LNUM}[\.][0-9]*) EXPONENT_DNUM ( ({LNUM} | {DNUM}) [eE][+-]? {LNUM}) The size of a float is platform-dependent, although a maximum of ~1.8e308 with a precision of roughly 14 decimal digits is a common value (that's 64 bit IEEE format). It is quite usual that simple decimal fractions like 0.1 or 0.7 cannot be converted into their internal binary counterparts without a little loss of precision. This can lead to confusing results: for example, floor((0.1+0.7)*10) will usually return 7 instead of the expected 8 as the result of the internal representation really being something like 7.9999999999.... This is related to the fact that it is impossible to exactly express some fractions in decimal notation with a finite number of digits. For instance, 1/3 in decimal form becomes 0.3333333. . .. So never trust floating number results to the last digit and never compare floating point numbers for equality. If you really need higher precision, you should use the arbitrary precision math functions or gmp functions instead. For information on when and how strings are converted to floats, see the section titled String conversion to numbers. For values of other types, the conversion is the same as if the value would have been converted to integer and then to float. See the Converting to integer section for more information. As of PHP 5, notice is thrown if you try to convert object to float. A string is series of characters. In PHP, a character is the same as a byte, that is, there are exactly 256 different characters possible. This also implies that PHP has no native support of Unicode. See utf8_encode and utf8_decode for some Unicode support. It is no problem for a string to become very large. There is no practical bound to the size of strings imposed by PHP, so there is no reason at all to worry about long strings. A string literal can be specified in three different ways. single quoted double quoted heredoc syntax The easiest way to specify a simple string is to enclose it in single quotes (the character '). To specify a literal single quote, you will need to escape it with a backslash (\), like in many other languages. If a backslash needs to occur before a single quote or at the end of the string, you need to double it. Note that if you try to escape any other character, the backslash will also be printed! So usually there is no need to escape the backslash itself. In PHP 3, a warning will be issued at the E_NOTICE level when this happens. Unlike the two other syntaxes, variables and escape sequences for special characters will not be expanded when they occur in single quoted strings. ]]> If the string is enclosed in double-quotes ("), PHP understands more escape sequences for special characters: sequence meaning \n linefeed (LF or 0x0A (10) in ASCII) \r carriage return (CR or 0x0D (13) in ASCII) \t horizontal tab (HT or 0x09 (9) in ASCII) \\ backslash \$ dollar sign \" double-quote \[0-7]{1,3} the sequence of characters matching the regular expression is a character in octal notation \x[0-9A-Fa-f]{1,2} the sequence of characters matching the regular expression is a character in hexadecimal notation Again, if you try to escape any other character, the backslash will be printed too! But the most important feature of double-quoted strings is the fact that variable names will be expanded. See string parsing for details. Another way to delimit strings is by using heredoc syntax ("<<<"). One should provide an identifier after <<<, then the string, and then the same identifier to close the quotation. The closing identifier must begin in the first column of the line. Also, the identifier used must follow the same naming rules as any other label in PHP: it must contain only alphanumeric characters and underscores, and must start with a non-digit character or underscore. It is very important to note that the line with the closing identifier contains no other characters, except possibly a semicolon (;). That means especially that the identifier may not be indented, and there may not be any spaces or tabs after or before the semicolon. It's also important to realize that the first character before the closing identifier must be a newline as defined by your operating system. This is \r on Macintosh for example. If this rule is broken and the closing identifier is not "clean" then it's not considered to be a closing identifier and PHP will continue looking for one. If in this case a proper closing identifier is not found then a parse error will result with the line number being at the end of the script. Heredoc text behaves just like a double-quoted string, without the double-quotes. This means that you do not need to escape quotes in your here docs, but you can still use the escape codes listed above. Variables are expanded, but the same care must be taken when expressing complex variables inside a here doc as with strings. foo = 'Foo'; $this->bar = array('Bar1', 'Bar2', 'Bar3'); } } $foo = new foo(); $name = 'MyName'; echo <<foo. Now, I am printing some {$foo->bar[1]}. This should print a capital 'A': \x41 EOT; ?> ]]> Heredoc support was added in PHP 4. When a string is specified in double quotes or with heredoc, variables are parsed within it. There are two types of syntax: a simple one and a complex one. The simple syntax is the most common and convenient. It provides a way to parse a variable, an array value, or an object property. The complex syntax was introduced in PHP 4, and can be recognised by the curly braces surrounding the expression. If a dollar sign ($) is encountered, the parser will greedily take as many tokens as possible to form a valid variable name. Enclose the variable name in curly braces if you want to explicitly specify the end of the name. ]]> Similarly, you can also have an array index or an object property parsed. With array indices, the closing square bracket (]) marks the end of the index. For object properties the same rules apply as to simple variables, though with object properties there doesn't exist a trick like the one with variables. 'red', 'banana' => 'yellow'); // Works but note that this works differently outside string-quotes echo "A banana is $fruits[banana]."; // Works echo "A banana is {$fruits['banana']}."; // Works but PHP looks for a constant named banana first // as described below. echo "A banana is {$fruits[banana]}."; // Won't work, use braces. This results in a parse error. echo "A banana is $fruits['banana']."; // Works echo "A banana is " . $fruits['banana'] . "."; // Works echo "This square is $square->width meters broad."; // Won't work. For a solution, see the complex syntax. echo "This square is $square->width00 centimeters broad."; ?> ]]> For anything more complex, you should use the complex syntax. This isn't called complex because the syntax is complex, but because you can include complex expressions this way. In fact, you can include any value that is in the namespace in strings with this syntax. You simply write the expression the same way as you would outside the string, and then include it in { and }. Since you can't escape '{', this syntax will only be recognised when the $ is immediately following the {. (Use "{\$" or "\{$" to get a literal "{$"). Some examples to make it clear: width}00 centimeters broad."; // Works echo "This works: {$arr[4][3]}"; // This is wrong for the same reason as $foo[bar] is wrong // outside a string. In other words, it will still work but // because PHP first looks for a constant named foo, it will // throw an error of level E_NOTICE (undefined constant). echo "This is wrong: {$arr[foo][3]}"; // Works. When using multi-dimensional arrays, always use // braces around arrays when inside of strings echo "This works: {$arr['foo'][3]}"; // Works. echo "This works: " . $arr['foo'][3]; echo "You can even write {$obj->values[3]->name}"; echo "This is the value of the var named $name: {${$name}}"; ?> ]]> Characters within strings may be accessed and modified by specifying the zero-based offset of the desired character after the string in curly braces. For backwards compatibility, you can still use array-brackets for the same purpose. However, this syntax is deprecated as of PHP 4. ]]> Strings may be concatenated using the '.' (dot) operator. Note that the '+' (addition) operator will not work for this. Please see String operators for more information. There are a lot of useful functions for string modification. See the string functions section for general functions, the regular expression functions for advanced find&replacing (in two tastes: Perl and POSIX extended). There are also functions for URL-strings, and functions to encrypt/decrypt strings (mcrypt and mhash). Finally, if you still didn't find what you're looking for, see also the character type functions. You can convert a value to a string using the (string) cast, or the strval function. String conversion is automatically done in the scope of an expression for you where a string is needed. This happens when you use the echo or print functions, or when you compare a variable value to a string. Reading the manual sections on Types and Type Juggling will make the following clearer. See also settype. A boolean &true; value is converted to the string "1", the &false; value is represented as "" (empty string). This way you can convert back and forth between boolean and string values. An integer or a floating point number (float) is converted to a string representing the number with its digits (including the exponent part for floating point numbers). Arrays are always converted to the string "Array", so you cannot dump out the contents of an array with echo or print to see what is inside them. To view one element, you'd do something like echo $arr['foo']. See below for tips on dumping/viewing the entire contents. Objects are always converted to the string "Object". If you would like to print out the member variable values of an object for debugging reasons, read the paragraphs below. If you would like to find out the class name of which an object is an instance of, use get_class. As of PHP 5, __toString() method is used if applicable. Resources are always converted to strings with the structure "Resource id #1" where 1 is the unique number of the resource assigned by PHP during runtime. If you would like to get the type of the resource, use get_resource_type. &null; is always converted to an empty string. As you can see above, printing out the arrays, objects or resources does not provide you any useful information about the values themselves. Look at the functions print_r and var_dump for better ways to print out values for debugging. You can also convert PHP values to strings to store them permanently. This method is called serialization, and can be done with the function serialize. You can also serialize PHP values to XML structures, if you have WDDX support in your PHP setup. When a string is evaluated as a numeric value, the resulting value and type are determined as follows. The string will evaluate as a float if it contains any of the characters '.', 'e', or 'E'. Otherwise, it will evaluate as an integer. The value is given by the initial portion of the string. If the string starts with valid numeric data, this will be the value used. Otherwise, the value will be 0 (zero). Valid numeric data is an optional sign, followed by one or more digits (optionally containing a decimal point), followed by an optional exponent. The exponent is an 'e' or 'E' followed by one or more digits. ]]> For more information on this conversion, see the Unix manual page for strtod(3). If you would like to test any of the examples in this section, you can cut and paste the examples and insert the following line to see for yourself what's going on: \n"; ?> ]]> Do not expect to get the code of one character by converting it to integer (as you would do in C for example). Use the functions ord and chr to convert between charcodes and characters. An array in PHP is actually an ordered map. A map is a type that maps values to keys. This type is optimized in several ways, so you can use it as a real array, or a list (vector), hashtable (which is an implementation of a map), dictionary, collection, stack, queue and probably more. Because you can have another PHP array as a value, you can also quite easily simulate trees. Explanation of those data structures is beyond the scope of this manual, but you'll find at least one example for each of them. For more information we refer you to external literature about this broad topic. An array can be created by the array language-construct. It takes a certain number of comma-separated key => value pairs. array( key => value , ... ) // key may be an integer or string // value may be any value "bar", 12 => true); echo $arr["foo"]; // bar echo $arr[12]; // 1 ?> ]]> A key may be either an integer or a string. If a key is the standard representation of an integer, it will be interpreted as such (i.e. "8" will be interpreted as 8, while "08" will be interpreted as "08"). There are no different indexed and associative array types in PHP; there is only one array type, which can both contain integer and string indices. A value can be of any PHP type. array(6 => 5, 13 => 9, "a" => 42)); echo $arr["somearray"][6]; // 5 echo $arr["somearray"][13]; // 9 echo $arr["somearray"]["a"]; // 42 ?> ]]> If you do not specify a key for a given value, then the maximum of the integer indices is taken, and the new key will be that maximum value + 1. If you specify a key that already has a value assigned to it, that value will be overwritten. 43, 32, 56, "b" => 12); // ...this array array(5 => 43, 6 => 32, 7 => 56, "b" => 12); ?> ]]> As of PHP 4.3.0, the index generation behaviour described above has changed. Now, if you append to an array in which the current maximum key is negative, then the next key created will be zero (0). Before, the new index would have been set to the largest existing key + 1, the same as positive indices are. Using &true; as a key will evaluate to integer 1 as key. Using &false; as a key will evaluate to integer 0 as key. Using NULL as a key will evaluate to the empty string. Using the empty string as key will create (or overwrite) a key with the empty string and its value; it is not the same as using empty brackets. You cannot use arrays or objects as keys. Doing so will result in a warning: Illegal offset type. You can also modify an existing array by explicitly setting values in it. This is done by assigning values to the array while specifying the key in brackets. You can also omit the key, add an empty pair of brackets ("[]") to the variable name in that case. $arr[key] = value; $arr[] = value; // key may be an integer or string // value may be any value If $arr doesn't exist yet, it will be created. So this is also an alternative way to specify an array. To change a certain value, just assign a new value to an element specified with its key. If you want to remove a key/value pair, you need to unset it. 1, 12 => 2); $arr[] = 56; // This is the same as $arr[13] = 56; // at this point of the script $arr["x"] = 42; // This adds a new element to // the array with key "x" unset($arr[5]); // This removes the element from the array unset($arr); // This deletes the whole array ?> ]]> As mentioned above, if you provide the brackets with no key specified, then the maximum of the existing integer indices is taken, and the new key will be that maximum value + 1 . If no integer indices exist yet, the key will be 0 (zero). If you specify a key that already has a value assigned to it, that value will be overwritten. As of PHP 4.3.0, the index generation behaviour described above has changed. Now, if you append to an array in which the current maximum key is negative, then the next key created will be zero (0). Before, the new index would have been set to the largest existing key + 1, the same as positive indices are. Note that the maximum integer key used for this need not currently exist in the array. It simply must have existed in the array at some time since the last time the array was re-indexed. The following example illustrates: $value) { unset($array[$i]); } print_r($array); // Append an item (note that the new key is 5, instead of 0 as you // might expect). $array[] = 6; print_r($array); // Re-index: $array = array_values($array); $array[] = 7; print_r($array); ?> ]]> The above example would produce the following output: 1 [1] => 2 [2] => 3 [3] => 4 [4] => 5 ) Array ( ) Array ( [5] => 6 ) Array ( [0] => 6 [1] => 7 ) ]]> There are quite a few useful functions for working with arrays. See the array functions section. The unset function allows unsetting keys of an array. Be aware that the array will NOT be reindexed. If you only use "usual integer indices" (starting from zero, increasing by one), you can achieve the reindex effect by using array_values. 'one', 2 => 'two', 3 => 'three'); unset($a[2]); /* will produce an array that would have been defined as $a = array(1 => 'one', 3 => 'three'); and NOT $a = array(1 => 'one', 2 =>'three'); */ $b = array_values($a); // Now $b is array(0 => 'one', 1 =>'three') ?> ]]> The foreach control structure exists specifically for arrays. It provides an easy way to traverse an array. You should always use quotes around a string literal array index. For example, use $foo['bar'] and not $foo[bar]. But why is $foo[bar] wrong? You might have seen the following syntax in old scripts: ]]> This is wrong, but it works. Then, why is it wrong? The reason is that this code has an undefined constant (bar) rather than a string ('bar' - notice the quotes), and PHP may in future define constants which, unfortunately for your code, have the same name. It works because PHP automatically converts a bare string (an unquoted string which does not correspond to any known symbol) into a string which contains the bare string. For instance, if there is no defined constant named bar, then PHP will substitute in the string 'bar' and use that. This does not mean to always quote the key. You do not want to quote keys which are constants or variables, as this will prevent PHP from interpreting them. ]]> The output from the above is: More examples to demonstrate this fact: 'apple', 'veggie' => 'carrot'); // Correct print $arr['fruit']; // apple print $arr['veggie']; // carrot // Incorrect. This works but also throws a PHP error of // level E_NOTICE because of an undefined constant named fruit // // Notice: Use of undefined constant fruit - assumed 'fruit' in... print $arr[fruit]; // apple // Let's define a constant to demonstrate what's going on. We // will assign value 'veggie' to a constant named fruit. define('fruit', 'veggie'); // Notice the difference now print $arr['fruit']; // apple print $arr[fruit]; // carrot // The following is okay as it's inside a string. Constants are not // looked for within strings so no E_NOTICE error here print "Hello $arr[fruit]"; // Hello apple // With one exception, braces surrounding arrays within strings // allows constants to be looked for print "Hello {$arr[fruit]}"; // Hello carrot print "Hello {$arr['fruit']}"; // Hello apple // This will not work, results in a parse error such as: // Parse error: parse error, expecting T_STRING' or T_VARIABLE' or T_NUM_STRING' // This of course applies to using autoglobals in strings as well print "Hello $arr['fruit']"; print "Hello $_GET['foo']"; // Concatenation is another option print "Hello " . $arr['fruit']; // Hello apple ?> ]]> When you turn error_reporting up to show E_NOTICE level errors (such as setting it to E_ALL) then you will see these errors. By default, error_reporting is turned down to not show them. As stated in the syntax section, there must be an expression between the square brackets ('[' and ']'). That means that you can write things like this: ]]> This is an example of using a function return value as the array index. PHP also knows about constants, as you may have seen the E_* ones before. ]]> Note that E_ERROR is also a valid identifier, just like bar in the first example. But the last example is in fact the same as writing: ]]> because E_ERROR equals 1, etc. As we already explained in the above examples, $foo[bar] still works but is wrong. It works, because bar is due to its syntax expected to be a constant expression. However, in this case no constant with the name bar exists. PHP now assumes that you meant bar literally, as the string "bar", but that you forgot to write the quotes. At some point in the future, the PHP team might want to add another constant or keyword, or you may introduce another constant into your application, and then you get in trouble. For example, you already cannot use the words empty and default this way, since they are special reserved keywords. To reiterate, inside a double-quoted string, it's valid to not surround array indexes with quotes so "$foo[bar]" is valid. See the above examples for details on why as well as the section on variable parsing in strings. For any of the types: integer, float, string, boolean and resource, if you convert a value to an array, you get an array with one element (with index 0), which is the scalar value you started with. If you convert an object to an array, you get the properties (member variables) of that object as the array's elements. The keys are the member variable names. If you convert a &null; value to an array, you get an empty array. It is possible to compare arrays by array_diff and by Array operators. The array type in PHP is very versatile, so here will be some examples to show you the full power of arrays. 'red', 'taste' => 'sweet', 'shape' => 'round', 'name' => 'apple', 4 // key will be 0 ); // is completely equivalent with $a['color'] = 'red'; $a['taste'] = 'sweet'; $a['shape'] = 'round'; $a['name'] = 'apple'; $a[] = 4; // key will be 0 $b[] = 'a'; $b[] = 'b'; $b[] = 'c'; // will result in the array array(0 => 'a' , 1 => 'b' , 2 => 'c'), // or simply array('a', 'b', 'c') ?> ]]> 4, 'OS' => 'Linux', 'lang' => 'english', 'short_tags' => true ); // strictly numerical keys $array = array( 7, 8, 0, 156, -10 ); // this is the same as array(0 => 7, 1 => 8, ...) $switching = array( 10, // key = 0 5 => 6, 3 => 7, 'a' => 4, 11, // key = 6 (maximum of integer-indices was 5) '8' => 2, // key = 8 (integer!) '02' => 77, // key = '02' 0 => 12 // the value 10 will be overwritten by 12 ); // empty array $empty = array(); ?> ]]> ]]> This will output: Note that it is currently not possible to change the values of the array directly in such a loop. A workaround is the following: $color) { // won't work: //$color = strtoupper($color); // works: $colors[$key] = strtoupper($color); } print_r($colors); ?> ]]> This will output: RED [1] => BLUE [2] => GREEN [3] => YELLOW ) ]]> This example creates a one-based array. 'January', 'February', 'March'); print_r($firstquarter); ?> ]]> This will output: 'January' [2] => 'February' [3] => 'March' ) ]]> ]]> Arrays are ordered. You can also change the order using various sorting functions. See the array functions section for more information. You can count the number of items in an array using the count function. ]]> Because the value of an array can be anything, it can also be another array. This way you can make recursive and multi-dimensional arrays. array ( "a" => "orange", "b" => "banana", "c" => "apple" ), "numbers" => array ( 1, 2, 3, 4, 5, 6 ), "holes" => array ( "first", 5 => "second", "third" ) ); // Some examples to address values in the array above echo $fruits["holes"][5]; // prints "second" echo $fruits["fruits"]["a"]; // prints "orange" unset($fruits["holes"][0]); // remove "first" // Create a new multi-dimensional array $juices["apple"]["green"] = "good"; ?> ]]> You should be aware that array assignment always involves value copying. You need to use the reference operator to copy an array by reference. ]]> To initialize an object, you use the new statement to instantiate the object to a variable. do_foo(); ?> ]]> For a full discussion, please read the section Classes and Objects. If an object is converted to an object, it is not modified. If a value of any other type is converted to an object, a new instance of the stdClass built in class is created. If the value was null, the new instance will be empty. For any other value, a member variable named scalar will contain the value. scalar; // outputs 'ciao' ?> ]]> A resource is a special variable, holding a reference to an external resource. Resources are created and used by special functions. See the appendix for a listing of all these functions and the corresponding resource types. The resource type was introduced in PHP 4 As resource types hold special handlers to opened files, database connections, image canvas areas and the like, you cannot convert any value to a resource. Due to the reference-counting system introduced with PHP 4's Zend Engine, it is automatically detected when a resource is no longer referred to (just like Java). When this is the case, all resources that were in use for this resource are made free by the garbage collector. For this reason, it is rarely ever necessary to free the memory manually by using some free_result function. Persistent database links are special, they are not destroyed by the garbage collector. See also the section about persistent connections. The special &null; value represents that a variable has no value. &null; is the only possible value of type NULL. The null type was introduced in PHP 4 A variable is considered to be &null; if it has been assigned the constant &null;. it has not been set to any value yet. it has been unset. There is only one value of type &null;, and that is the case-insensitive keyword &null;. ]]> See also is_null and unset. mixed indicates that a parameter may accept multiple (but not necessarily all) types. gettype for example will accept all PHP types, while str_replace will accept strings and arrays. number indicates that a parameter can be either integer or float. Some functions like call_user_func or usort accept user defined callback functions as a parameter. Callback functions can not only be simple functions but also object methods including static class methods. A PHP function is simply passed by its name as a string. You can pass any builtin or user defined function with the exception of array, echo, empty, eval, exit, isset, list, print and unset. A method of an instantiated object is passed as an array containing an object as the element with index 0 and a method name as the element with index 1. Static class methods can also be passed without instantiating an object of that class by passing the class name instead of an object as the element with index 0. ]]> PHP does not require (or support) explicit type definition in variable declaration; a variable's type is determined by the context in which that variable is used. That is to say, if you assign a string value to variable $var, $var becomes a string. If you then assign an integer value to $var, it becomes an integer. An example of PHP's automatic type conversion is the addition operator '+'. If any of the operands is a float, then all operands are evaluated as floats, and the result will be a float. Otherwise, the operands will be interpreted as integers, and the result will also be an integer. Note that this does NOT change the types of the operands themselves; the only change is in how the operands are evaluated. ]]> If the last two examples above seem odd, see String conversion to numbers. If you wish to force a variable to be evaluated as a certain type, see the section on Type casting. If you wish to change the type of a variable, see settype. If you would like to test any of the examples in this section, you can use the var_dump function. The behaviour of an automatic conversion to array is currently undefined. ]]> Since PHP (for historical reasons) supports indexing into strings via offsets using the same syntax as array indexing, the example above leads to a problem: should $a become an array with its first element being "f", or should "f" become the first character of the string $a? The current versions of PHP interpret the second assignment as a string offset identification, so $a becomes "f", the result of this automatic conversion however should be considered undefined. PHP 4 introduced the new curly bracket syntax to access characters in string, use this syntax instead of the one presented above: ]]> See the section titled String access by character for more information. Type casting in PHP works much as it does in C: the name of the desired type is written in parentheses before the variable which is to be cast. ]]> The casts allowed are: (int), (integer) - cast to integer (bool), (boolean) - cast to boolean (float), (double), (real) - cast to float (string) - cast to string (array) - cast to array (object) - cast to object Note that tabs and spaces are allowed inside the parentheses, so the following are functionally equivalent: ]]> Instead of casting a variable to string, you can also enclose the variable in double quotes. ]]> It may not be obvious exactly what will happen when casting between certain types. For more info, see these sections: Converting to boolean Converting to integer Converting to float Converting to string Converting to array Converting to object Converting to resource The type comparison tables